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Probability theory is naturally much involved. This is unsurprising in the context of theory and model construction. But in Part three ‘Entropy and Time’s Arrow’ chapter 10 ‘Recurring Nightmare’ the role of probability theory seems less straightforward.

Statistical Mechanics relies on the Principle of Indifference – the assumption that all microstates consistent with our current macrostate are equally likely, at least when it comes to predicting the future. That’s essentially an assumption of typicality: Our microstate is likely to be a typical member of our macrostate. If we’re not allowed to assume that, all sorts of statistical reasoning become inaccessible.

That is, the universe must be a nail, because we know how to use a hammer.